1. Field of the Invention
This invention relates to power shovels, and more particularly to hydraulic mechanisms for operating a sliding dipper handle of the mining shovel.
2. Description of the Related Art
A typical mining power shovel includes a turntable mounted on a crawler truck, and supporting an A-frame and a cab. A boom, extending from the turntable, has an upper end supported by the A-frame and pivotally supporting a dipper handle that pivots in a vertical plane. A dipper fixed to a distal end of the dipper handle is raised and lowered by a hoist cable which extends over a sheave at the top of the boom and down to a padlock on the dipper. The hoist cable provides for the vertical, raising and lowering, movement of the dipper. A crowd assembly extends and retracts the dipper handle to provide the horizontal component, or crowd, of the dipper's movement.
Many different crowd assemblies have been developed over the years. Rack and pinion crowd assemblies include a rack fixed to the dipper handle which engages a rotatably driven pinion, or gear, mounted in the boom. Rope crowd assemblies include metal ropes that are wound and unwound from a crowd drum to extend and retract the dipper handle.
Also known in the art are hydraulic crowd assemblies, which utilize a large double-acting hydraulic actuator. U.S. Pat. No. 3,425,574 discloses a hydraulic crowd assembly that has a saddle block comprising a tubular support frame pivotally coupled by a yoke to the boom. A round tubular dipper handle, attached to the dipper, slides into and out of the support frame as the dipper moves with respect to the boom. That sliding motion is driven by a double acting hydraulic actuator comprising a cylinder within which a piston moves in response to pressurized hydraulic fluid being fed into the cylinder. A piston rod, connected to the piston, projects outward through an aperture at one end of the cylinder. The other end of the cylinder is attached to an extremity of the support frame that is remote from the saddle block and the exterior end of the piston rod is connected to a section of the dipper handle that is remote from the support frame. Thus the hydraulic actuator is supported at one end by connection of the cylinder to the support frame and at the other end by attachment of the piston rod to the dipper handle. Except at those ends, there are no supports between the either the hydraulic actuator and either the dipper handle or the saddle block.
Because hydraulic actuator of the dipper handle is very long, especially when the piston rod is extended significantly from the cylinder, and because the crowd assembly and its hydraulic actuator often extend substantially horizontal, the combination of the cylinder and piston rod can deflect or sag due to gravity. As a result, the rod frequently passes off center through the aperture in the cylinder, resulting in a smaller gap in the cylinder aperture above the rod than beneath the rod. In fact, the upper part of the piston rod can contact the cylinder, thereby scoring the surfaces sliding on each other. Over time, this deflection and the piston rod scoring can damage the aperture seal between the piston rod and the cylinder, resulting in leakage of hydraulic oil.